LUNG VOLUMES AND CAPACITIES
17.2.1 Lung Volumes
1. Tidal volume: Volume of air inspired or expired in each respiratory cycle during rest = 500 ml.
2. Inspiratory reserve volume: Maximal volume of air which can be inspired after normal inspiration = 3000 ml
3.
Expiratory reserve volume: Maximal volume of air that can be expired after normal expiration = 1000 ml.4. Residual volume: Volume of air remaining in the lung after maximal expiration i.e. it cannot be expelled to the atmosphere except after opening the chest = 1000 - 1500 ml.
a) Significance of residual volume:
b) Maintains the aeration of blood between breaths.
c) Diagnosis of respiratory diseases: R.V. is normally below 30% of total lung capacity in adults. It increases and may reach 70% in conditions associated with difficult expiration as:
1) Bronchoconstriction, e.g. bronchial asthma.
2) Weak elasticity of lungs, e.g., emphysema.
d) In these conditions the lungs do not normally collapse during expiration. They remain more distended than normal.
5. Minimal air: the volume of air remains in the lungs after opening the thorax and the complete collapse of the lungs.
Significance of minimal air: important in forensic medicine to know whether an infant was born alive then killed or was born dead. If a piece of its lung is:
• Float on water, indicating that minimal air is present. It proves that infant was born alive then killed.
• Not floats on water indicating that minimal air is absent. It proves that infant was born dead.
17.2.1.1 Lung Capacities (Summation of Two or More Lung Volumes Together)
• Inspiratory capacity (maximal volume of air which can be inspired after normal inspiration) = Tidal volume (500 ml) + Inspiratory reserve volume (3000 ml).
• Functional residual capacity (volume of air remaining in lungs after normal expiration, this volume is known as resting expiratory level)
= Expiratory reserve volume (1000 ml) + Residual volume (1200 ml)
• Total lung capacity: (volume of air contained in the lungs after deepest inspiration)
= All lung volume = 5700 ml
=Tidal volume (500 ml) + Inspiratory reserve volume (3000 ml) + Expiratory reserve volume (1000 ml) + Residual volume (1200 ml).
• Vital capacity: (Volume of air given out by maximal expiration after maximal inspiration).
= Total lung capacity (5700 ml) - Residual volume (1200 ml) = 4500 ml.
N.B. It is better measured in relation to body surface area (s.a.). So, it is 2.5 litres/m2 s.a. in adult males and 2.0 litres/m2 s.a. in adult females.
Significance of vital capacity (VC): when calculated / mm2 s.a., VC is an index of pulmonary ventilation efficiency and physical fitness.
17.2.1.2 Factors Affecting V.C.
1. Conditions of respiratory muscles:
a) Strong, e.g., athletes; lung distension is more efficient and increase V.C.
b) Difficult movement, e.g., spinal cord injuries, ascites, late pregnancy, poliomyelitis. The lung distension is not efficient and decrease V.C
2. Posture:
a) When sitting or standing, the viscera drop down by gravity and the diaphragm so lung distension is more efficient and increase V.C.
b) In the recumbent position, the viscera press on the diaphragm, resulting in lung distension that is not efficient and decreases V.C.
3. Decrease lung elasticity:
e.g., T.B., emphysema → lung does not distend efficiently → decrease V.C.
4. Decrease expansion and recoil of the thoracic cage:
e.g. deformities of the chest cage, weak respiratory muscles, pneumonia → decrease V.C.
5. Increase volume of blood (stagnation) in pulmonary circulation:
e.g. left-side heart failure → decreased lung dis- tensibility and excess blood takes the space for air → decreased V.C.
6. Increase resistance to airflow in respiratory passages:
e.g. bronchial asthma → decrease V.C
17.2.1.3 Dead Space (D.S.)
It is the space of a conducting passage, e.g., nasal cavity, pharynx, trachea, bronchi, and bronchioles, in which there is no gas exchange.
17.2.1.4 Measurement of D.S. (Using Bohr's Equation) CO2 % in alveolar air - CO2 % in expired air with measurement of tidal volume.
It is about 150 ml at the start of expiration.17.2.1.5 Significance of D.S
1. Protects lungs against damage:
a) Moisten and warm inspired air.
b) Filter and clean inspired air from large particles.
2. Responsible for the difference between:
a) Composition of alveolar and expired air.
b) Total and true (effective) alveolar ventilation, this is important in conditions of shallow rapid respiration as a large part of tidal volume occupies the D.S → decrease of effective ventilation.
3. One of the pulmonary function tests.
N.B.: Dead Space
Increase
• Sympathetic stimulation.
• During inspiration due to widening of air passage.
• Old age.
Decrease
• Vagus stimulation.
• During expiration due to constriction of air passage.
17.2.1.6 Respiratory Rate of Different Animals
Respiratory rate means the number of respirations per minute. It is affected by several factors.
1. Animal species:
Human: 12-20
Horse: 9-10
Cow and buffalo: 12-16
Camel: 6-8
2. Body size: There is an inverse relationship between respiratory rate and animal size.
3. Exercise: Muscular exercise increases respiratory rate.
4. Environmental temperature: High environmental temperature increases respiratory rate.
5. Excitement: Fear and fright increase the respiratory rate.
6. Diseases: Fever, cardiac, and respiratory diseases increase respiratory rate.
17.2.1.7 Respiratory Sounds (Ralls)
The inlet and outlet of air through the respiratory passages can be heard by the stethoscope over different parts of the chest wall. Diseases of the respiratory system lead to changes in the normal respiratory sound; vesicular-V-sound.
17.3.1